Description of the Prior Art
Operational amplifiers (op amps) are commonly used to provide voltage gain. One known type of op amp is a differential-input differential-output op amp which is characterized by higher power supply rejection, higher signal swing and higher signal-to-noise ratio with lower harmonic distortion than the commonly used differential-input single-ended output op amp.
In addition to the external differential mode feedback, differential op amps require an internal common-mode feedback circuit. To this end, the sum of two op amp outputs must be amplified and fed back to the common-mode input of the op amp. One method for summing the two outputs uses an MOS differential pair, having a limited differential swing for the main op amp, because the linear range of the differential pair employed in the common-mode feedback circuit is severely limited. A second technique employs switched capacitor summing circuits, which is limited to sampled data circuits such as SC filters. This type of circuit is difficult to simulate during the design cycle of the op amp. A third technique uses resistors and requires a two stage op amp utilizing a common source type output stage. The op amp is compensated by a pair of series resistor-capacitors across the two stages in order to provide satisfactory open loop gain and driving capability across the summing resistors. Due to the limited bandwidth of the second stage, the bandwidth of the first stage is intentionally reduced through a compensation capacitor. Phase margin is improved by a capacitive load dependent pole cancellation, and thus the range of load capacitance is limited. The resulting op amp has a wide range of applications and can be used in continuous time circuits such as MOSFET-C, anti-aliasing and smoothing filters, as well as SC filters.
An alternative approach to the use of resistors in a two stage op amp utilizes operational transconductance amplifiers (OTA) buffered by a pair of source followers. Source followers have a wide bandwidth, close to unity gain and a low output impedance. Therefore, the overall gain is not adversely affected when the circuit is loaded by the summing resistors of the common-mode feedback circuit. In this type of circuit, the bandwidth of the overall op amp is close to that of the input OTA. This type of op amp makes use of a pair of single-ended compensation capacitors connected between the two high impedance outputs of the OTA and one of the power supplies.
Another disadvantage of presently known differential op amps is that they require a relatively large chip area.